The present invention relates to a test apparatus for determining control constants of a PID controller by an autotuning or automatic tuning method.
Heretofore, as a fatigue test machine wherein loads, such as tensile load and compressive load, are continuously applied to a specimen or test piece for a long time to thereby test a fatigue strength of the specimen, there has been known a test machine wherein an actuator for applying loads to the specimen is controlled by a PID controller through a feedback so that waves of displacement of the specimen or waves of the loads applied to the specimen, i.e. waves of an output signal, coincide with waves of a request signal, i.e. input signal, inputted at a predetermined cycle.
In the fatigue test machine, although control constants, such as proportional gain K, integral time TI and derivative time TD, of the PID control are fixed, the control constants are set to sufficiently small values so that the output signal is not unstabilized by changes of conditions of a system, such as changes of characteristics of the specimen and changes of circumferential conditions; more specifically, for example, hunting does not occur in output waves even in case rigidity of the specimen is changed by the loads during the test.
However, as described above, if the control constants of the PID control are simply set to the small values, response degree of the output waves is deteriorated especially in a high frequency area, and line forming ability of the output signal against the input signal is deteriorated. Therefore, there is carried out a control method, i.e. autotuning method, for changing the control constants of the PID control based on the changes of conditions of the system during the test.
In the autotuning method, there is employed a control method wherein, for example, a parameter for showing conditions of the system is determined by sampling data of output signals with respect to input signals over a plurality of cycles during a test, and the control constants of the PID control are changed based on a theoretical equation from the parameter. However, in the control method, samplings of the data over the plural cycles are required, which takes a long time and is not suitable for the fatigue test.
Also, as another example of the autotuning method, there has been known, so called, a noise method. In the noise method, instead of sampling data over the plural cycles, a noise component is intentionally added to an input signal, and a parameter for showing conditions of a system is determined according to a response with respect to the noise component. The noise method is superior in that the parameter is determined for every cycle. However, since the noise component contained in the input signal acts as a load suddenly applied to the specimen, a bad influence may be exerted to the specimen. This becomes a problem especially when a noise strength is increased in order to improve accuracy of parameter determination.
The present invention has been made in view of the above described defects, and an object of the invention is to provide a test apparatus, wherein control constants of a PID control can be optimized for every cycle.
Another object of the invention is to provide a test apparatus as stated above, wherein a bad influence is not exerted to a specimen in the optimizing course of the control constants.
Further objects and advantages of the invention will be apparent from the following description of the invention.